Many metal ducts and tubes having very irregular bends, bulges and indentations are used in commercial aircraft. These are principally used to duct air from one location to another. The most harsh environment, and the one requiring the most convoluted metal tubes, is the ducting of hot exhaust gases from jet engines.
Some such ducts which are relatively straight and not harshly bulged or indented are shaped by conventional bulge forming methods. U.S. Pat. No. 2,372,917 to Tuttle shows a conventional method of bulge forming a tube in a split female tool. A liquid is pressurized in the interior of the tube while pressure is vented from the tool cavities. German 24 42 801 to Seizo also shows a bulge forming method featuring an intermediate fluid filled chamber to accomplish pressurization of the forming liquid. U.S. Pat. No. 3,564,886 shows a similar bulge-forming method in a vented tool. U.S. Pat. Nos. 3,359,624 and 3,462,821, both to Cours et al., are of general interest as background for pipe forming using certain bulge forming techniques.
Prior art FIG. 1 shows a conventional bulge forming apparatus 1. Apparatus I consists of an upper platen 2 and lower platen 4. Jig collar 6 holds first die half 8 and second die half 10 together with tube 12 therebetween. Tube 12 is formed to match interior contour 14 of dies 8 and 10 by pushing bottom a piston 16 and top piston 22 together. An incompressible fluid 20 is fed into to 12 through inlet 18 in piston 16. Air is simultaneously evacuated through outlet 24 in top piston 22. When all the air is gone, a valve 26 on outlet 24 is closed and pressure is applied to fluid 20 causing tube 12 to bulge into dies 8 and 10. While this method is effective, it has a number of limitations. It requires a high tonnage press and high strength machined tools that will withstand the application of hydraulic pressures up to 20,000 psi. While the method may be cost effective where thousands of parts are to be made, it is very expensive to provide a high tonnage press and such tooling for short run pads. Moreover, the process generally lends itself only to the manufacture of rather straight tubes of rather short length and small interior volume. The method also generates substantial scrap above and below the bulge formed section of pipe.
Most part runs of large tubes and ducts for commercial airplanes are short, forming has conventionally been done by hammer forming small segments and then welding them together. While tooling costs for this method are relatively low, it is very labor intensive and difficult to control quality.
It is the object of this invention to provide novel, isostatic bulge forming equipment and methods which overcome the shortcomings of this prior art.